Heretofore, methods for producing fibrous wollastonite (hereafter referred to as "wollastonite") have been known, as described, for example, in U.S. Pat. Nos. 3,799,836 and 4,060,401.
In accordance with these methods, a bar-like base glass is produced and heated up to a predetermined temperature progressively from the edge portion thereof by a heater to crystallize wollastonite crystals in the glass, the heater is moved according to the crystallization to produce a mixture of bundles of wollastonite crystals and a glass matrix phase, and the bar-like crystallized glass is pulverized to obtain crystallized glass containing needles of wollastonite therein.
In any of the crystallized glasses containing wollastonite crystals produced by the above methods, when the crystallized glass is used as a reinforcing fibrous material, it is necessary to disintegrate the wollastonite present in the crystallized glass into individual units. Therefore, the crystallized glass is pulverized to obtain wollastonite. In the pulverization, however, cleavage easily occurs along the cleavage plane of the wollastonite crystal, but breakage also occurs at areas other than the cleavage plane. Therefore, the aspect ratio (ratio of length in a longitudinal direction to length in a vertical direction) is smaller than in the crystallized glass.
With these crystallized glasses, therefore, no great reinforcing effect as a reinforcing material can be expected.
Further, with the glass compositions described above, the yield of wollastonite from the base glass is low, a heat-treatment for crystallization of the base glass requires very high precision, and efficiency of production is low.